Method of forming niobium doped tin oxide coatings on glass and coated glass formed thereby

ABSTRACT

A niobium doped tin oxide coating is applied onto a glass substrate to produce a low emissivity (low E) glass. The coating can optionally be doped with both niobium and other dopant(s), such as fluorine. The low emissivity glass has properties comparable or superior to conventional low E glass with fluorine doped tin oxide coatings.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation application claiming priorityfrom U.S. patent application Ser. No. 09/534,863, which was filed onMar. 24, 2000. U.S. patent application Ser. No. 09/534,863 was pendingas of the filing date of the present application. U.S. patentapplication Ser. No. 09/534,863 is hereby expressly incorporated byreference as if set forth in its entirety herein.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a film suitable for use as a coating ona glass substrate. More particularly, this invention is directed to aniobium doped tin oxide coating applied onto a glass substrate toproduce a low emissivity (low E) glass.

[0003] Coatings on glass are commonly utilized to provide specificenergy attenuation and light transmittance properties. Additionally,coatings are designed to reduce reflections from interfaces betweenindividual coating layers and the glass when a plurality of coatings areapplied onto a glass substrate. The coated articles are often utilizedsingularly, or in combination with other coated articles, to form aglazing.

[0004] The attributes of the resulting coated glass substrate aredependent upon the specific coatings applied to the glass substrate. Thecoating compositions and thicknesses impart energy absorption and lighttransmittance properties within the coated article while also affectingthe spectral properties. Desired attributes may be obtainable byadjusting the compositions or thicknesses of the coating layer orlayers.

[0005] A particular variety of glass product commonly used in thebuilding industry is low E glass. A primary advantage of the low E glassis that it provides superior thermal control properties, i.e. it limitsthe passing of thermal energy (infrared wavelengths) through the glass,while maintaining high transmission of visible light. Low E glass can beproduced through a sputter coating (soft coat) or preferably, through apyrolytic process, for example chemical vapor deposition. Typically,glass produced through a pyrolytic process yields a coating which isless easily damaged and less likely to deteriorate under exposure toair.

[0006] Low E glass has significant uses in building products and otherapplications such as substrates for solar cells, display panels, heatedrefrigerated displays and computer screens. The lower the E value of theglass the less thermal energy is transmitted through the glass andtherefore the easier it becomes to control the internal temperature of abuilding equipped with the low E glass. In many applications, the low Eglass can also preferably be color neutral. This allows viewing throughthe glass with a minimal amount of color distortion. Low E glass can beused alone, or in combination with additional panes of tinted glass orreflective glass to obtain different appearances and thermal controlproperties.

[0007] Coatings with sheet resistance value less than about 500 ohms persquare are generally considered to be conductive coatings. Theemissivity of a coated glass article is directly related to its sheetresistance. By lowering the sheet resistance, or increasing theconductivity, of a glass sheet, the emissivity is reduced.

[0008] In theory, a coating of pure tin oxide on a glass substrate wouldhave an extremely high sheet resistance. However, in practice, tin oxidecoatings typically have a sheet resistance of about 350-400 ohms persquare. This is due, at least in part, to an oxygen deficiency in thetin oxide, rendering it at least slightly conductive. Fluorine is oftenused as a tin oxide dopant in order to increase the conductivity. Afluorine doped tin oxide coating (SnO₂:F) can produce sheet resistancesas low as about 16 Ω/cm². When tin oxide is doped with fluorine, thefluorine will substitute for oxygen in the compound. This substitutionof fluorine for oxygen is a factor in the lowered sheet resistance, dueto their differing electron configurations. Other materials have beenalso used as dopants in various glass coating applications.

[0009] It would be advantageous to use a material as a dopant, alone orin combination with fluorine or other dopants, which results in acoating having a comparable or lowered emissivity for a given thickness,while maintaining or improving the ease and cost of manufacture of thecoated glass products, and without impairing the optical qualities ofthe glass.

SUMMARY OF THE INVENTION

[0010] In accordance with the present invention, there is provided afilm suitable for use as a coating on glass. The film is a niobium dopedtin oxide which can be produced by combining a niobium source withconventional tin oxide deposition precursors. The amount of niobiumpresent in the film is variable based on the planned application. Thecoating of the present invention can also be a tin oxide coating dopedwith both niobium and other known dopant(s), such as fluorine. Thecoating of the present invention can thus be used as an alternative to,or in conjunction with, fluorine doped tin oxide coatings for glasssubstrates, especially for use in low E glass.

[0011] The present invention further provides a process of making acoated glass sheet, preferably by a pyrolytic process, for example bychemical vapor deposition (CVD), wherein the coating includes a niobiumdoped layer of tin oxide, or optionally, a layer of tin oxide dual dopedwith niobium and fluorine.

[0012] The Nb doped coating of the present invention can be used as asingle layer on a glass substrate, or in conjunction with other possibleembodiments of the present invention, may be used as a layer in amulti-layer coating stack. In possible embodiments of the presentinvention, Nb can be used as the sole dopant for the SnO₂ coating, or inthe alternative, Nb can be used in conjunction with other dopants, suchas fluorine.

[0013] The niobium doped tin oxide can preferably be appliedpyrolytically, on-line onto a float glass ribbon, by a process such aschemical vapor deposition which is well known in the art.

[0014] It is an object of the present invention to provide a coatedglass material having an emissivity comparable to or lower than theemissivity of known coated glass products.

[0015] Another object of the present invention is to provide a method ofmaking a coated glass article with a reduced emissivity.

[0016] It is a further object of the present invention to provide aconductive film that can be pyrolytically deposited onto a glasssubstrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] The niobium doped tin oxide of the present invention is suitablefor use with conventional tin oxide deposition precursors. The pyrolyticdeposition enables the application of the film onto a float glass ribbondirectly in the glass production process, preferably by CVD.

[0018] Glass substrates suitable for use in preparing the coated glassarticle according to the present invention may include any of theconventional clear glass compositions known in the art. The preferredsubstrate is a clear float glass ribbon wherein the coating of thepresent invention, possibly with other optional coatings, is applied inthe heated zone of the float glass process. However, other conventionalprocesses for applying coatings on glass substrates are suitable for usewith the present inventive coating.

[0019] For a pyrolytic deposition, the doped tin oxide alloy isdeposited onto the glass substrate by incorporating a niobium sourcewith conventional tin oxide precursors. An example would include the useof niobium pentachloride (NbCl₅) in an inert gas, such as helium. TheNbCl₅ is a solid at normal atmospheric temperatures and pressures. Thus,for use as a dopant in the CVD process, the niobium pentachloride isvaporized and injected into a gas stream. A bubbler could be used, butin production conditions it would be preferable to use equipment such asa thin film evaporator to get the niobium pentachloride into the gasstream. Other possible niobium containing compounds are possible withinthe scope of the present invention. A significant factor in theselection of the niobium containing material is its volatility.Typically, the Nb containing material should be volatile at temperaturesbetween 0 and 500° F., and in a preferred embodiment of the presentinvention, the Nb containing material should be volatile within thetemperature range of 300-500° F. Niobium pentachloride is recommendedboth for its low melting point and because it is readily commerciallyavailable, however the present invention is intended to incorporate anyknown niobium compound suitable for doping tin oxide.

[0020] If the tin oxide were to be doped with, for example, fluorine andniobium, a fluorine source would also then be used with the conventionaltin oxide precursors. A preferred fluorine source would be either HF ortrifluoroacetic acid (TFA), but other conventional fluorine sourcescould be incorporated within the scope of the invention.

[0021] Tin precursors for glass coating processes are conventional andwell known in the art. An especially suitable tin containing compound isdimethyltin dichloride (DMT). This substance is well known and readilyavailable, and is commonly used as a tin precursor material in knownfloat glass coating applications. Other known tin precursors are alsousable within the scope of the present invention.

[0022] In at least one possible process of carrying out the presentinvention, NbCl₅ and DMT are run through thin film evaporators and arethen mixed with oxygen and water in a helium carrier gas. The oxygen canbe provided in the form of elemental oxygen or in the form of air,depending on the process employed. Other oxygen containing materials arecertainly usable within the scope of the process, but it is generallymost economical to use either air or elemental oxygen. The optionalfluorine containing material (preferably HF) would also be added iffluorine doping was desired. The precursor materials can then beintroduced into a coater which directs the materials to the surface of afloat glass ribbon. Care must be taken in the introduction of thematerials however, as premature reaction of the NbCls and water arepossible. A niobium doped tin oxide film is then deposited on a floatglass ribbon by conventional chemical vapor deposition techniques.

[0023] In the event that fluorine and niobium are being added in a dualdoping system, the fluorine precursor and the H₂O can be run through thesame thin film evaporator, although this is not necessary.

[0024] As opposed to conventional fluorine doping of tin oxide, whereinthe fluorine atoms replace oxygen, the niobium atoms replace tin atomsin the tin oxide layer. Niobium is especially suited to this as it has asimilar outer shell electron configuration to tin (5 electrons in theouter shell), and has an atomic number comparable to that of tin.Therefore, it is theorized that the niobium easily takes the place ofthe tin atoms in the tin oxide.

[0025] It has surprisingly been found that doping with niobium alone canyield similar sheet resistance properties to doping with fluorine.However, it has unexpectedly been found that doping with both fluorineand niobium can yield sheet resistances superior to doping with eitherniobium or fluorine alone.

[0026] The following examples, which constitute the best mode presentlycontemplated by the inventors for practicing the present invention, arepresented solely for the purpose of further illustrating and disclosingthe present invention, and are not to be construed as a limitation onthe invention:

[0027] The following is an explanation of the data listed in Tables 1-4below:

[0028] DMT represents the flow of dimethyltin dichloride in standardliters per minute;

[0029] H₂O represents the flow of water in standard cm³ per minute;

[0030] O₂ represents the oxygen flowrate in standard liters per minute;

[0031] NbCl₅ represents the niobium pentachloride flow suspended inhelium in standard liters per minute;

[0032] TFA is the flow of trifluoroacetic acid in milliliters per hour(which was maintained at 0 for samples 1-5 to provide fluorine freesamples);

[0033] SHR represents the sheet resistance in ohms per square; and

[0034] thickness is the calculated thickness in Angstroms based on colormeasurements.

SAMPLES 1-5

[0035] A first set of samples were produced using a conventional processand doped only with niobium. In the samples, dimethyltin dichloride wasused as the tin source, and H₂O and O₂ were also added. NbCl₅ was runthrough a bubbler and suspended in a helium carrier gas. Additionalinert gas (He) was added to obtain the desired overall flow rate. Thesheet resistance of the resultant coating was measured by the use of afour point probe. The thickness of the resultant coating was analyzed bycolor analysis. Alternatively, profilometry techniques could have beenused to analyze the thickness of the coating.

[0036] Sample 1 included a DMT flow rate of 0.6 standard liters perminute (slm.) H₂O was added at a rate of 10.3 standard cm³ per minute.O₂ was added at the rate of 18 standard liters per minute. Sample 1 wasa baseline test sample containing no niobium pentachloride. Sample 1produce a sheet resistance of about 350-400 ohms/square and a thicknesscalculated at about 3800 angstroms. The remaining samples generatedsheet resistances between 16 and 35 Ω/cm² and calculated thicknessesbetween 3700 and 4200 A. The sheet resistance test results for theniobium doped tin oxide were similar to expected results for fluorinedoped tin oxide coatings of similar thickness. The results from tests1-5 are summarized in Table 1. TABLE 1 sample DMT H₂O O₂ NbCl₅ TFA SHRthickness 1 0.6 10.3 18 0 0 350-400 3800 2 0.6 10.3 18 0.5 0 25 3800 30.6 10.3 18 0.25 0 35 3700 4 0.6 10.3 18 1 0 16 4200 5 0.6 10.3 18 0.1 020 4200

SAMPLES 6-12

[0037] For samples 7-12, the tin oxide layer was doped with both niobiumand fluorine. The TFA and H₂O were run through the same thin filmevaporator for these tests. Sample 6 was tested with only a fluorinedopant as a comparative baseline for the tests. The DMT, H₂O and O₂flowrates were held constant through the tests, although at differentlevels than in samples 1-5.

[0038] It can be seen from Table 2 that the sheet resistances werecomparable to that obtained from fluorine doping, but in at least onecase the coating had a superior sheet resistance to that of a fluorinedoped tin oxide layer alone. TABLE 2 sample DMT H₂O O₂ NbCl₅ TFA SHRthickness 6 1.26 7.2 18 0 59.73 14 3200-3700 7 1.26 7.2 18 0.25 59.73 202500-3100 8 1.26 7.2 18 0.25 59.73 12 3300-3800 9 1.26 7.2 18 0.35 59.7314 3300-3800 10 1.26 7.2 18 0.15 59.73 14 3300-3800 11 1.26 7.2 18 0.3540 14 2800-3300 12 1.26 7.2 18 0.35 70 16 2800-3300

SAMPLES 13-18

[0039] Samples 13-18 were again run with no fluorine containingcompound, but in these samples the water and NbCl₅ concentrations wereboth varied. As can be seen in the accompanying Table 3, varying sheetresistances were obtained depending on the sample conditions. TABLE 3sample DMT H₂O O₂ NbCl₅ TFA SHR thickness 13 10 10 5.2 0 0 1500 2800 141 10 5.2 0.5 0 115 4400 15 1 10 5.2 0.2 0 50 4700 16 1 6 5.2 0.2 0 504700 17 1 3 5.2 0.2 0 43 4400 18 1 3 5.2 2.1 0 120 4200

SAMPLES 19-32

[0040] Additional samples doped with only niobium were prepared and runwith two different concentrations of DMT, while varying the amount ofNbCl₅ supplied. Additionally, more samples were run without dopants, inorder to obtain additional comparative data. It can again be seen, inthe accompanying Table 4, that significant improvements in sheetresistances were obtained by doping with Nb compared to the undopedsamples. TABLE 4 sample DMT H₂O O₂ NbCl₅ TFA SHR thickness 19 0.64 10 180 0 700 1800 20 0.64 10 18 0 0 500 1800 21 0.64 10 18 0.25 0 50-70 180022 0.64 10 18 0.35 0 60-70 1200-1500 23 0.64 10 18 0.45 0 60-80 1200 240.64 10 18 0.15 0 120-140 1200-1500 25 0.64 10 18 0 0 400 2600 26 1 1018 0 0 400-700 3000 27 1 10 18 0 0 450-800 3200 28 1 10 18 0.25 0 35-403200 29 1 10 18 0.35 0 30-35 3200 30 1 10 18 0.55 0 30-35 2000-2500 31 110 18 1 0 35-40 1800-2100 32 1 10 18 0.12 0 60-70 3200

[0041] One further advantage noted from the niobium doped samples wasnoted when testing the Hall effect. The Hall effect was tested byrunning a current along the longitudinal axis of the sample. A magneticfield was then run perpendicular to the plane of the sample, generatingan induced current across the sample, i.e. perpendicular to thedirection of applied current flow. It is known that the induced voltageis a function of the number of electrons carrying current (n_(e)) andthe mobility (σ) of the material. The highest known n_(e) value reportedin the literature for fluorine doped tin oxide is about 5.5×10²⁰electrons per cubic centimeter (e⁻/cm³.) Testing of sample 8 indicatedan electron concentration of about 7−8×10²⁰ e⁻/cm³, which is asignificant increase from the values reported in the literature. Thenumber of electrons carrying current is inversely proportional to thesheet resistance and the emissivity, therefore indicating that thehigher number of electrons available to carry current in the niobiumdoped tin oxide could lead to even lower emissivity and sheet resistancevalues, and is a further sign of the improved characteristics of theniobium doped tin oxide coating on glass.

[0042] In accordance with the provisions of the patent statutes, thepresent invention has been described in what is considered to representits preferred embodiment. However, it should be noted that the inventioncan be practiced otherwise than as specifically illustrated anddescribed without departing from its spirit or scope. For example, othercoating methods, such as sputtering, may also be utilized to form thepyrolytic coating of the present invention.

What is claimed is:
 1. A method of making a coated glass article comprising the steps of: providing a glass substrate at an elevated temperature, the glass substrate having a surface on which the coating is to be deposited; forming a precursor mixture including a tin containing compound, an oxygen containing compound, an inert carrier gas and a niobium containing compound and directing the precursor mixture at and along the surface to be coated to form a coating on the surface of the glass substrate; and cooling the coated glass substrate to ambient temperature.
 2. The method according to claim 1 wherein the niobium containing compound is niobium pentachloride.
 3. The method according to claim 1 wherein the precursor mixture further includes a fluorine containing compound.
 4. The method according to claim 3 wherein the fluorine containing compound is selected from the group consisting of hydrogen fluoride and trifluoroacetic acid.
 5. The method according to claim 1 wherein the precursor mixture includes H₂O.
 6. The method according to claim 1 wherein the coating is formed to a thickness of between 2500 and 3800 Å.
 7. A method of making a coated glass article comprising the steps of: providing a glass substrate having a surface on which a coating is to be deposited; directing a tin containing compound, an oxygen containing compound and a niobium containing compound to the surface on which the coating is to be deposited to form a coating on the surface of the glass substrate.
 8. The method according to claim 7 wherein the niobium containing compound is niobium pentachloride.
 9. The method according to claim 7 wherein the precursor mixture further includes a fluorine containing compound.
 10. The method according to claim 9 wherein the fluorine containing compound is selected from the group consisting of hydrogen fluoride and trifluoroacetic acid.
 11. The method according to claim 7 wherein the precursor mixture includes H₂O.
 12. The method according to claim 7 wherein the coating is formed to a thickness of between 2500 and 3800 Å. 